Abernathyite, a new uranium mineral from the Fuemrol No. 2 mine, Emery County, Utah, has the formula K(UO2)(AsO4)·4H2O. The mineral occurs as transparent, yellow, fluorescent, thick tabular crystals belonging to the tetragonal system, ditetragonal-di-pyramidal class (4/m 2/m 2/m). The space group is P4/nmm; a0 = 7.17 ± 0.01 Å; c0 = 9.08 ± 0.01 Å; a:c = 1:1.266; Z = 2. Optically, the mineral is uniaxial negative, ϵ = 1.570 ± 0.003, ω = 1.597 ± 0.003. The hardness is between 2 and 3. The calculated specific gravity is 3.74. The chemical analysis shows, inpercent:K2O,9.5; UO3, 57.7; As2O5, 21.6; P2O5,1.5; H2O —, 4.6;H2O +, 9.9; total, 104.8. Abernathyite is named for the finder, Jess Abernathy, operator of the Fuemrol mine.